1. Field of the Invention
This invention relates generally to a powersplit powertrain for a hybrid electric vehicle.
2. Description of the Prior Art
The powertrain of a hybrid electric vehicle (HEV) includes an engine, at least one electric machine which can act as a motor/generator and traction battery, wherein the engine and an electric machine can drive the wheels individually, the engine can charge the traction battery through an electric machine operating as a generator, and vehicle kinetic energy can be recovered and regenerated during deceleration by powering an electric machine to act as a generator to recharge the battery.
A powersplit powertrain provides excellent fuel economy during city driving, but incurs a penalty during highway driving due to negative split operation. Grounding the generator using a clutch does not produce the best speed ratio for the engine under normal highway driving conditions.
A powersplit powertrain does not have a dedicated reverse gear, but rather produces reverse operation using the electric motor only to provide torque. Due to this operation, the reverse launch capability is directly related to the peak motor torque, which is usually less than a conventional vehicle. The peak motor torque is thus a key design parameter for the powersplit hybrid system.
Using a generator brake, it is possible to engage the generator brake during highway speed, thereby eliminating the transmission losses incurred during negative split operation. However, due to the gear ratios, this typically establishes an engine speed that is too high for optimal fuel consumption. The use of a generator brake in practice is limited to a small operating region.
A powersplit powertrain that includes a one-way clutch on the engine input shaft allows the generator to contribute torque to improve the peak output torque during a forward launch. However, this has been found to cause a large “torque hole” decrease in output shaft torque when the engine is started. In addition, the one-way clutch can not be used to aid in reverse launch because the torque direction does not cause the clutch to engage.